1. Field of the Invention
The present invention relates to a continuous casting nozzle for permitting effective prevention of narrowing or clogging of the nozzle bore through which molten steel passes from tundish to mold while performing continuous casting of molten steel containing aluminum such as aluminum-killed steel.
2. Description of the Related Art
A continuous casting nozzle for casting molten steel is used for the following purposes. A continuous casting nozzle, which is a means for feeding molten steel from a tundish to a mold, is used for the purpose of preventing the molten steel from being oxidized when contacting the open air and from splashing when the molten steel is poured from a tundish to a mold, and for the purpose of rectifying the flow of the molten steel poured for preventing non-metallic inclusion and slag present near or on the mold surface from being entrapped in the cast steel strand.
Material of a conventional continuous casting nozzle for casting molten steel comprises such material as graphite, alumina, silica, silicon carbide and recently zirconia. However, there are following problems in the case of casting aluminum-killed steel and the like.
As for the aluminum-killed steel and the like, aluminum, which is added as a de-oxidizer to molten steel, reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as alpha-alumina. Therefore, in casting the aluminum-killed steel and the like, the non-metallic inclusion such as alpha-alumina adheres and accumulates onto the surface of the bore of the continuous casting nozzle, so that the bore is narrowed or clogged in the worst case, which makes stable casting difficult. Furthermore, the non-metallic inclusion such as alpha-alumnina, adhered or accumulated onto the surface of the bore, peels off or falls down, and is entrapped in the cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned narrowing or clogging of the bore caused by the non-metallic inclusion such as alpha-alumina, there is a commonly used method for preventing the non-metallic inclusion such as alpha-alumina existing in the molten steel from adhering or accumulating on the surface of the bore of the nozzle by ejecting inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore. An example of this method is described in Japanese Patent Publication No. Hei 6-59533/1994. However, the following for the above-mentioned method wherein inert gas is ejected from the inner surface of the nozzle.
A large amount of the ejected inert gas causes entrapment of bubbles produced by the inert gas in the cast steel strand, resulting in defects based on pinholes. On the other hand, a small amount of the ejected inert gas causes adhesion and accumulation of the non-metallic inclusion such as alpha-alumina onto the surface of the bore of the nozzle, thus causing narrowing or clogging, in the worst case, of the bore.
Additionally, it is constructionally difficult to uniformly eject the inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore. Also, in the case that the casting is performed over a long period of time, a stable control of the amount of ejected inert gas becomes gradually more difficult as the composition and the structure of the material consisting of the continuous casting nozzle degrades. Moreover, it becomes difficult to eject inert gas uniformly from the inner surface to the nozzle bore. As a result, the non-metallic inclusion such as alpha-alumina adheres and accumulates onto the surface of the bore of the nozzle so that the bore is narrowed or clogged at the end.
It is thought that the clogging of the nozzle by the non-metallic inclusion, especially by the alumina inclusion, is caused as described below.
(1) Alumina inclusion is produced from aluminum in the steel by secondary oxidation, such as oxidation by entrapped air passing through a refractory junction and refractory structure. PA1 (2) In addition, silica contained in conventional alumina-graphite material of a nozzle reacts with the graphite to produce silicon-monoxide(SiO), which oxidizes aluminum in steel to produce alumina. PA1 (3) Alumina inclusion is produced by diffusion and cohesion of the alumina particles produced in the above process. PA1 (4) Graphite on the surface of the nozzle bore vanishes and the surface of the bore becomes rough thus, the alumina inclusion is apt to accumulate on the rough surface of the bore.
On the other hand, as a counterplan in view of nozzle material, a alumina-graphite nozzle containing a non-oxide raw material (SiC, Si.sub.3 N.sub.4, BN, ZrB.sub.2, SIALON, etc.) that has low reactivity with aluminum oxide, or a nozzle consisting of the non-oxide material itself is proposed. An example of this type of counterplan is described in Japanese Patent Publication No. Sho 61-38152/1986. However, this counterplan is not practical in the case of the alumina-graphite nozzle because the adhesion preventing effect is not recognized and further corrosion resistance is decreased unless much of the non-oxide material is added.
Also, a nozzle consisting of only the non-oxide material is not suitable for practical use in view of material cost and manufacturing cost, although a substantial effect is expected.
A nozzle consisting of graphite-oxide raw material containing CaO has been proposed. A low-melting-point material is produced by a reaction of CaO in the oxide raw material containing CaO (CaO.ZrO.sub.2, CaO.S.sub.i O.sub.2, 2CaO.S.sub.i O.sub.2 etc.) with Al.sub.2 O.sub.3, which is easily separated from the steel. An example of this is described in Japanese Patent Laid-Open Publication No. Sho 62-56101/1987.
However, the reactivity of CaO with Al.sub.2 O.sub.3 is apt to be influenced by a temperature condition of the molten steel in casting, and there is a case in which the amount of CaO is not sufficiently secured for satisfying spalling resistance and corrosion resistance when high levels of Al.sub.2 O.sub.3 inclusion is contained in the steel.